The present invention relates to a molded dosimeter that provides for accurate and simple measurement of the absorbed doses of ionization radiations such as gamma-rays, X-rays, electron beams, neutron rays and heavily charged particle rays. The invention is intended for expanding the use of an alanine dosimeter.
Recent years have seen a rapid increase in the number of large facilities that handle radioactive substances (e.g. nuclear power plants and radioactive waste treating plants) and particle accelerators that produce a variety of ionizing radiations such as particle rays and gamma-rays. In these facilities and particle accelerators, it is required to provide for accurate and simple radiation dosimetry over a broad range that includes not only ordinary environments but also hot and humid environments. The present invention will provide great benefits for dosimetry conducted at these commercial facilities and at laboratories that use a variety of radiations for research and experimental purposes, as well as in dosimetric comparisons between particle accelerators.
Various solid-state dosimeters are known that are intended for measuring at medium: doses ranging from 10 Gy to 100 kGy, i.e. high level of dose, and they include thermoluminescent dosimeters, lyoluminescent dosimeters, polymethyl methacrylate dosimeters, radiochromic die film dosimeters and cobalt glass dosimeters. The operating principles of these dosimeters are similar to that after irradiating the solid-state device with a radiation of interest, the amount of luminescence emitted from the device by heating or the amount of light with a specific wavelength absorbed by the device is measured for determining the quantity of radiation that has fallen on the device.
These conventional dosimeters have the following defects: (1) Except for glass dosimeters, considerable dispersions will occur in dosimetric response characteristics (e.g. amount of luminesence from the device and the amount of light absorbed by the device) even if it is irradiated with a radiation of interest under the same conditions within the same environment; (2) Except for thermoluminescent dosimeters and radiochromic die film dosimeters, the conventional devices are subject to fading occurs, or a phenomenon wherein a time-dependent change occurs in the dosimetric response after irradiation of interest; (3) A narrow dynamic range over which accurate dosimetry can be made; and (4) Radiachromic die film dosimeters and lyoluminescent dosimeters are subject to considerable dispersions in dosimetric response resulting from changes in environmental factors such as temperature and humidity during irradiation of interest.
When the crystal of alanine, an amino acid, is irradiated with a radiation, it produces stable and characteristic radicals (free radicals) in precise proportion to the dose absorbed by the crystal, which can, therefore, be determined by measuring the concentration of radicals in unit weight of the crystal with a paramagnetic electron spin resonance (esr) instrument (CEA-R-3913, France, 1970). This method of dosimetry has none of the problems involved in the other conventional dosimeters described above. The radicals formed by irradiation with radiation remain stable within the crystal of analine and their concentration is subjected to a very small degree of time-dependent change. For the same reason, the radicals formed in the crystal of alanine are fairly stable against heat. Therefore, this method enables dosimetry to be performed with high precision and reproducibility. In addition, the dynamic range of the method is from 10 Gy and 100 kGy, permitting more reliable dosimetry at medium to high levels than any other conventional dosimeters.
The powder is so fine that the great tendency of the powder to attract static charges will present considerable difficulty in measuring its weight accurately or inserting it into a sample tube, i.e., it is very difficult to handle it. For this reason, the powder of analine crystal alone does not have much utility as a commercial dosimeter.
A few studies have been made in order to develop dosimeters that makes the most of the advantages of alanine crystal while eliminating any of the defects it has in powder form. Methods that hve been proposed in these studies and which are currently considered to be standard depend on using paraffin or a cellulose powder as a binder agent; the powder of alanine crystal is well dispersed in a melted paraffin, and the mixture is cooled to solidify with stirring, and then compressed into a pellet for use as a dosimetric element, or the powder of alanine crystal is mixed with cellulose powder and the mixture is compressed into a pellet for a dosimetric element (Inter. J. Appln. Radt. Isotope, 33, 1101 (1982), and Rad. Protection, EUR 7448-EN, vol. 12, 489 (1982)). However, the crystal analine pellets pressed together with paraffin or cellulose gives a brittle product which will easily break pieces or crumble upon external forces, and impacts thereby requiring to make careful treatment to accomplish accurate dosimetry. Furthermore, these pellets must be done by compression (for paraffin or cellulose), so the pressed products that can be obtained are limited to either pellets or short cylinders. It is practically impossible to realize mass production of these alanine crystal pellet having uniform properties by the above described method of using paraffin or cellulose. Further problems arise from the inherent properties of the binder agents used. Paraffin has a maximum melting point of about 70.degree. C., so the alanine crystal pellet using paraffin as a binder agenft cannot be used in high-temperature environments such as where a metal container is irradiated with high dose rates. On the other hand, cellulose itself produces peroxide radiclas of which esr signals overlap these of radicals formed in the alanine crystal and make it difficult to accomplish esr determination of the accurate radical concentration due only to the analine crystal. Therefore, dosimetry with the alanine crystal pressed together with cellulose as a binder agent provides inaccurate results and the dynamic range of the device is narrower than that achieved by using the alanine crystal alone. Another problem occurs in the case of using cellulose as a binder agent; the cellulose is used as a powder which cannot be well mixed with the alanine powder to provide a uniform composition, and the resulting pressed products have considerable dispersion in composition.